Effects of phosphate and 1,25(OH)2D3 on in vitro bone collagen synthesis in the hypophosphatemic mouse

Calvarial bones from hypophosphatemic (Hyp) mice and normal littermates were cultured in a chemically defined medium to determine: (a) the effect of medium phosphate (Pi) concentration (1, 2, and 3 mM) on collagen synthesis; (b) the effect of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] (10(-12)M-10(-7)M) on collagen synthesis; and (c) whether bone responsiveness to 1,25(OH)2D3 was affected by changes in medium Pi concentration. Bone collagen synthesis was evaluated by measuring [ 3H ]hydroxyproline formation. The distribution of labeled hydroxyproline between bone explant and culture medium (total and dialyzable fraction) was studied. These experiments confirm that 1,25(OH)2D3 inhibits specifically bone collagen synthesis in vitro. We did not detect any effect of medium Pi concentration on basal collagen synthesis but were able to demonstrate that lowering medium Pi concentration increased the 1,25(OH)2D3-induced inhibition of collagen synthesis. Bones from both genotypes responded to 1,25(OH)2D3, but modulation of this response by changes in Pi concentration was altered in Hyp bone as, in contrast to normal bone, its response to 1,25(OH)2D3 was unaffected when medium Pi concentration was decreased from 3 to 2 mM. These findings support the hypothesis of an altered response of bone to 1,25(OH)2D3 in the Hyp mouse.